The present invention relates to an electric plug capable of preventing a tracking of a main body of the plug made of a synthetic resin material.
As shown in FIG. 7, an electric plug or electric power supply plug 1 comprises a pair of electrically conductive blades 3, 3 respectively electrically connected to the tip ends of electrical wires of a cord or cable 2, a plug body 4 formed by extrusion-molding process of a synthetic resin material so as to allow the front portions of the parallel-blades 3, 3 to become exposed, and a bellows 5, and generally the plug body 4 and the bellows 5 are integrally formed by a thermoplastic polyvinyl chloride resin material (PVC).
However, since the electric plug made of PVC is considered to lack the resistance against tracking (the formation of a carbonized electrically conductive path due to a short-circuiting through dust or water-drops and an electric breakdown resulting therefrom), it is said to be unsuitable for use with strict specifications (for example, the 250V and 20A condition).
One of proposals for eliminating such problem is disclosed in the specification of the Japanese Utility Model No. 3021727. The attachment plug disclosed in this specification is of the type having a structure such that it has a plurality of terminals (corresponding to the parallel-electrically conductive blades) extending from the front face portion of the plug main body and that a thermosetting resin portion having a suitable thickness, which is hardly carbonized, is formed on the entire front face of the plug main body and a concave portion for increasing the creeping distance along an outer surface of the front face portion between the opposing surfaces of the terminals is formed.
Apart from the attachment plug disclosed in the above-described specification, there is another proposal disclosed in the Japanese Patent Laid-Open Publication No. H8-236189 such that a space filling means is provided at the root portions of a pair of electrically connecting terminals (electrically conductive blades) that extend from the plug main body so as to increase the above-described creeping distance between the terminals. According to this structure, about a half of each of the electrically connecting terminals (blades) is practically covered with a resin material.
However, the above-described prior art electric power supply plugs have the disadvantage that since the main body of each of the electric plugs is made up of the thermosetting resin portion and the soft synthetic resin portion, the manufacturing process becomes complicated and both of the resin portions lack the integrality thereof. Further, the structure in which about one half of each of the plug bodies is covered with a synthetic resin material has a serious problem in productivity with respect to its yield because the plug blades are required to be processed within the range prescribed by the industrial standard.